Interference-reducing system



June 3, 1952 wEN YuAN PAN 2,598,935

INTERFERENcE-REDUCING SYSTEM 2 SHEETS-SHEET 1 Filed Sept. 30, 1948INVENTOR WENYUAN FAN ATTORNEY June 3, 1952 wEN YUAN PAN 2,598,935

INTERFERENcE-REDUCING SYSTEM Filed Sept. 50, 1948 2 SHEETS-SHEET 2INVENTOR WEN YUAN FAN ATTORNEY d Patented June 3, 1952INTERFERENCE-REDUCING SYSTEM Wen Yuan Pan, Collingswood, N. J., assignorto Radio Corporation of America, a corporation of Delaware ApplicationSeptember 30, 1948, Serial No. 51,974

(Cl. Z50-20) 7 Claims. 1

This invention relates to multi-band heterodyne type signal receivingapparatus and more particularly to such apparatus having novelinterference-reducing features.

In the normal heterodyne type signal receivers such as radio receivingsets for use in homes, a signal-passing circuit is tuned to selectivelypass incoming signal waves of a desired frequency while awave-generating circuit is simultaneously tuned to cause the generationof mixer waves having a diiierent frequency. The signal and mixer Wavesare heterodyned or mixed together to form beat signals within apredetermined or so-called intermediate frequency (IF) channel foramplication by a high-eiliciency amplifying system having a fixedband-pass response. One important disadvantage of heterodyne receivershas been the response due to the heterodyning of the mixer waves withundesired so-called image signals having frequencies so related to thoseof the mixer waves that they also produce beat signals within thepredetermined channel. This is especially marked where the image signalsare of high intensity and are easily passed by the signal-passingcircuit even though this circuit may be tuned to attenuate them.

Among the objects of the present invention is the provision ofheterodyne type signal receivers having novel image-attenuatingfeatures.

Other objects of the invention include the provision of multi-bandheterodyne type signal receivers in which the tuning elements of oneband are utilized to attenuate image-frequency signals for another band.

A still further object of the invention is the provision cf multi-bandheterodyne type signal receivers in which tuning reactances for resonantcircuits of different bands are, in the interest of simplicity, linkedfor simultaneous tuning wheth er or not the circuits are switched intoreceiving operation and advantage is taken .of rthis simul taneoustuning over an unused band to selectively attenuate image signals in asignal-receiving band. g y i he above as well as other objects of theinvention will be more readily understoodfrom the following descriptionof exemplicationsthereof, reference being had to the accompanyingdrawings wherein:A

Fig. 1 is a circuit diagram ci a tuning section of a multi-band radiosignal receiving apparatus illustrating one modiiication of theinvention; and

Figs. 2, 3 and 4 are circuit diagrams, with parts omitted, showing theessential details of the 2 band-shifting arrangement of the constructionof Fig. 1.

According to the invention, multi-band heterodyne type signal receivershaving linked "or ganged tunable reactance elements for the separatebands are provided with switching structure so connected that as thedesired reactance elements are switched into the conventional receivingcircuits, at least one other tunable re'- actance is switched to form atuned image-attenuating network. By this technique, a signal receivingsystem can have its image rejection considerably increased at verylittle additional cost, and the increase can be eiective over a Widetuning range.

Fig. 1 shows the circuit construction of oneexample of the tuner sectionin a radio signal-receiving system embodying the invention. An antennaIis arranged for coupling, by means of combination switch I2, with anydesired one of a number of selective signal-passing circuits I4A, [4Band I4C. The switch i2 also connects'the desired signal-passing circuitwith the signal input of a mixer or heterodyne stage 20 which may be ofany convenient type. In the form shown the mixer stage includes anelectron-discharge pentagrid converter tube 22 having anelectronemissive cathode 24, an electron collecting plate 26 and vegrids 3l, 32, 33, 34 and 35 between the cathode and plate. The rst grid3| isvconnected as an oscillator grid by means of the wavegenerating oroscillator network 4d including series capacitors 42, 44 which areshunted across a desired one of a number of variable inductors 46-A,46-B, 46-C` to form a parallel resonantoscillation-frequency-determining or tank circuit. Small adjustablecapacitances l1-A, 4'l-B, lil-C are provided for connection across therespective inductors 15S-A, 46-B and 46-C` to fix their tuning ranges inrelation to the signal selecting circuits. The tankcircuit is connectedat one endto the oscillator grid 3| by means of coupling capacitor 48and at the other end to the grids 32 and 34 by means of the groundconnection and the by-pass capacitor 50. AV tap between the tankcondensers 42, 44 is connected yto Vthe cathode 24. The necessary D.C.electrode paths are established by an oscillator grid resistor 52, andcathode choke 54 both returning to ground. The grids 32, 34 which arenormally operated at a relatively large positive D.C. potential withrespect to the cathode are effectively held at ground potential for A.C.oscillations because of the bypass capacitor 5l), and in thiscombination, as is Well known, oscillations will be developed in thejpaeitors al, s2, s3.

oscillator tank and will appear on the oscillator grid 3|.

Grid 33 is connected as the signal input grid and is led to the signalselecting and passing circuit through a blocking capacitor 63 and tubeinput lead 6|. The D.C. return for grid 33 is provided by resistor 62which may be connected to a D.-C. reference potential source, forexample directly to ground or to the source of bias potential 54, suchas a conventional automatic volume control circuit for establishing thebias voltage between the grid and cathode and thereby controlling theeffect of the signal grid on the converting action of the tube 22.

Grid 35 is shown as merely connected to the cathode 24 for improving theelectron flow between cathode and plate. The positive potential forgrids 32, 34 may be supplied by a D.C. source, not shown, the positivelead to which is represented by the B+ sign, the negative lead beinggrounded. i

With the above arrangement, when there 1s I applied to the plate 26 apotential which is positive with respect to the cathode 24, varyingsignals appearing on the signal grid 33 and oscillations at theoscillator grid 3| will both affect the electron flow in tube 22 andcause the electron flow to vary in accordance with the frequency of thesignal variations and the oscillations as well .as beats, or sums anddifferences of these fre- .amplifiers in manners well known to thoseskilled in the art.

The selective circuits Ill-A, lli-B, |4-C are shown as parallel resonantcircuits including respectively variable inductors 1|, 12, 13 and ca-The signal-selective inductors 1|, 12, 13 as well as the wave-generatinginductors llt-A, 4BAB,'46-C are shown as arranged to have theirinductance values varied by means `of. movable high permeabilitymagnetictuning members 80 placed in the magneticeld space of the individualinductors. One practical inductance-varying arrangement is produced byforming the magnetic members 80 as cores which are movably mounted so asto penetrate to varying extents within the turns of the windings that wform Vthe corresponding inductors.

For simplicity of. construction the magnetic tuning members B are linkedor ganged together, as indicated by the dash lines so that all thevariable inductances are simultaneously varied regardless of whichparticular band is selected for signal reception. This is provided by asimple construction in which all the tuning members 80 are held on asingle control unit for simultaneous movement. Thus no complicatedshifting of tuning member linkages is needed as the apparatus isswitched to the diierent bands.

For assuring that tuning of the oscillation frequency faithfully tracksthe tuning of the signalpassing circuit, so that the heterodyne signalsstay in a fixed channel, the magnetic members have their relativepositions adjustable or pre-set. h

non-conductive disc 84 and engageable with a set of contacts l-l to l-ISby rotation of a band selector knob E5, fixed to the disc 84, as bymeans of a common shaft (not shown). A stationary portion of theapparatus may include frequency band legends shown as A, B and C and theknob may include a pointer 85 to indicate the specific band for whichthe switch is set. Stops 81, 8E provide the desired limits for themovement of the switch.

The contacts l-l to i-l are all secured to a nxed non-conductive waferS4 and project to different extents in the path of their correspondingcontact strip which is shaped to make the proper contact engagements.Contact I-| is connected to the antenna through coupling capacitor Il.Another capacitor i3 provides an impedance connection from the antennalead to ground and is selected lor cooperating with capacitor II to forman impedance matching coupling between the antenna and the signal inputof the converter stage 23.

Contacts l-2, I-S and l-li are connected to the high potential ends ofthe selective signal circuits Ill-A, lil-B and |4-C respectively. Thelow potential end of circuit lli-A is grounded, whereas the othercircuits |4-B and I ll-C have their inductors and capacitors separatelyconnected at their low potential ends to contacts l--3, #-45 and l-i/i,I-Ii respectively. Through additional reactance elements i35, ,95, S1,98 the low potential ends of these inductors and capacitors are alsoseparately connected to contacts i-ii, I-1, I-- and l-l respectively.

Oscillator inductor 4G-B has its upper and lower ends connected tocontacts |-.-5 and l-ll respectively. Contact l-S is the signal inputlead for converter stage 20 ,and contact l-I2 is the common signalreturn shown as a ground connection.

Another switch assembly 2I2, which may be constructed similarly to theswitch combination I2, is provided with apair or" movable contact strips5I, 53 and separate sets of fixed contacts 2-I to 2-1. Contact 2-Iisgrounded while contacts 2-2 and 2-1 are connected to the upper end ofoscillator inductance i6-B, and contact 2-3 is connected throughcapacitor 51 to the lower end of this inductance. The upper ends ofinductances i6-A and A16-C are separately led to contacts 2-4 and 2-1respectively. Contact 2--5 establishes the oscillator tank connectionbetween the tank capacitors 42, 44 and the selected tank inductance.

In the form of the invention shown, switches l2 and 2-|2 are ganged asby mechanical link 15. The contact strips are so shaped and the contactsof such length, that the following cperation is eected:

In the position in which the switches are illustrated in Fig. 1, theapparatus is set for reception in the A band. The circuit connectionsestablished by the switches are more clearly shown in simplified Fig. 2.-Antenna supply contact is connected' to ground across tuning circuitI4-A by means of contact strip 9| and contact |-2. At the same timecontact strip 9| with contacts l-l, and |-3, together with contact strip92 and contacts |-6, |-1 and |-9 establish an image attenuating tunedparallel-resonant circuit consisting of inductances 12, and capacitors82, 96, in series with the signal supply to the tube input lead 6|.Contact strip 53 by means of contacts 24, 2 5 inserts A-band oscillatorinductance I6-A into to thatfof the undesired images of the A band,"

In this'example the A-band oscillator tank 42, 44, 46`A rvls'adjusted togenerate waves uniformly higher in'frequenoy than the signals passed by"A band circuit I4A so that the converter outputV is in a substantiallyconstant frequency channelcorresponding to this difference in frequency.`With such operation, the undesired irnage sigrialsfa're higher infrequency than the generated waves,'by the same amountthat the generatedwaves are higher than the desired signals. Additionally the frequencyrange of the undesired image signal is smaller than the frequency rangeof the A band itself. In those constructions in which the differentsignal bands are tuned to cover approximately the same number ofoctaves, vthe range of tuned circuit III-B would have to be diminishedto follow the desired signal tuning of circuit I4-A and provide thedesired image attenuation. The addition of inductance 95, which is nottuned with the inductance gang, acts to minimize the inductance changesof inductance 12. Where the undesired image spectrum for "A band tuningis not coincident with the minimized tuning range of inductances 12, 95and the connected capacitance 82, the image tuning range is shifted asby the addition'of capacitance 95. As an exampleA of the A-bandoperation, and without limiting the invention in any way, the followingconstants are given:

A-band signal range, 540 to 1'720 kilocycles per second.

A-.band oscillator tuning range, 995 to 2175 kilocyclesper second. p

Image interference with A band, 1450 to 2630 kilocycles per second.

Converter output (IF center frequency),

kilocycles per second.

B-band signal range, 2.25 to 7.3 megacycles per second.

thelegend .B Thecontact vstripshll, 93, 5I and 53 are thereby shifted toestablish the circuits as shown in Pig. 3. Strip 9| which remains inengagement with its long antenna contact I-I becomes disconnected fromcontact I--2, retains-connection with contact I--S and becomesadditionally connected with contact I-4. Strip 92 simultaneouslydisengages the converted input contact I-B from contact I-6 and I-1 andconnects it instead with-contacts I-8and I-Iil. Strip 93 meanwhilegrounds contacts I-II, `I-Ii and I--I5. The lower end of adjustingcapacitor 41-B also becomes grounded, through strip 5I, and strip 53shifts the connection of oscillator tank contact 2-5 to contact 2 6.

As a result of the above circuit changes, tuned circuit I-B has itslower ends grounded and its upper end connected to antenna couplingcondenser I I, while the capacitance 83 and variable inductance 13 ofcircuit Ill-C become connected together with adjusting reactances 91, 98in series With the input lead 6I. At the same time oscillator inductance46-B as well as its trimmer capacitance i6-B, have their lower endsgrounded and upper ends connected in the oscillator tank circuit.Accordingly incoming signals will now be selected by tuning of circuit Id-B while images attenuation will be simultaneously tuned by thecombination of reactances 13, 83, 91, S8.

For reception of signals in the C band, the switches are rotated anotherstep in the clockwise direction, bringing indicator 8B to the legend Cand the circuits to the condition shown more clearly in Fig. 4. Theantenna thereby becomes connected to contacts I-IL I'5, by strip 9|while the lower ends of reactances 13, 83 are grounded by strip 93 toprovide the signal-selecting circuit. Image attenuation is establishedby the connection of oscillator windings IIG-B, between contacts I-5 andI-I I, in series with the input lead 6I to the antenna. The capacitance51 is selected to move the tuning range of inductance 46-B to thedesired image-rejection region. Oscillator tuning is provided by theconnection of windings I4-C in the oscillator tank circuit throughcontacts 2-5 and 2--1.

It will be found, when the desired signal range in one band is suitablyadjusted with respect to the intermediate frequency as well as thesignal range of another band, that the tunable oscillator tank reactancefor one band will have a reactance range suitable for directly trackingwith the image signals of the other band, even when both bands have thesame tuning range ratio. Thus, for example, and not by way oflimitation, withthe intermediate frequency 455 kilocycles per second,ranges may be selected as follows:

Band

Signal tuning Oscillator tuning Imago Range Range mc./sccond Rangemc./sccond i Rat o rnc/second Ratio the image-attenuating network.

For receiving signals in the B band, the switches I2 and 2-I2 are merelyrotated one step in the clockwise direction as by manually turning knob85 till the indicatorBB points to made'to exactly follow the'imagefrequency range of bands. Bands I or 2 of the second example given mayor may not be the same bands as those identified as A and B of theearlier example.

In the construction of Fig. 1 the resonant circuit inductances are tunedso that the capacitance across the oscillator windings are adjusted forimage rejection. In many cases the distributed capacity inherent in thewindings i6-B can be adjusted to the proper value for the desired imageattenuation so that merely disconnecting these windings from theoscillator tank circuit will suiilce to place them in image followingcondition. With such a constuction capacitor 51 and its associatedcontact may be omitted. Also, the trimmer capacitor 41-B may bepermanently connected across inducta'nce i6-B and the entire switchingof contact strip 5i eliminated.

If desired the image attenuation for band C may be provided by one ofthe other tuned circuits such as the A or B band signal selectingcircuit ifi-A or lli-B. When the B band oscillator is tuned tofrequencies higher than the B band signals, its tuning range is closestto the image frequencies of the higher band so that the reactanceelements of this oscillator are more closely suited to eiiicient imageattenuation for these very high frequency images. However, where theoscillator for a band is tuned to generate waves having frequenciesbelow the signal frequencies, the signal-selective circuit rather thanthe oscillator circuit of this band is slightly better suited for therejection of images of higher frequency bands. This is generallyapplicable because even adjacent bands usually have a spacing largerthan two times the oscillator frequency. Correspondingly for imageattenuation with respect to signals in bands of lower frequency, ahigher band resonant circuit having the lowest tuning range is generallymore effective.

A feature of the invention is the simplicity of construction. Inasmuchas all the resonant circuits are arranged for tuning whether or not theyare switched into operation, the construction of the invention onlyrequires the addition of a few fixed adjusting reactances and suitableswitching connections, all of which is relatively inexpensive. The addedadjusting reactances need not bevariable since accurate tracking in theimage rejection circuit is not necessary. Also with oscillatorsoperating at the high frequency side of the signal-selecting circuit,the image range ratio is much lower than the signal range and isadequately covered even with appreciable misalignment. This can bereadily seen from the A-band data given above. If desired however,variable adjusting reactances may be used, as for example when theintermediate frequency is much lower than the signal frequency and theimage range ratio is almost as large as that of the signal range, Seethe data for band 2, for example.

In those receiver constructions having permeability tuning of resonantcircuit inductances, it is normally the practice to provide separatelytuned inductances for each resonant circuit and gang the tuningtogether. This is exactly the manner suited for the invention.

It is obvious that the switching arrangement may be varied in manyrespects from that shown in the gure. For example different kinds ofswitches may be employed; each contact strip ill) may be embodied in aseparate ganged switch, all the contact strips may be on a single rotaryassembly such as that indicated at I2 having the necessary increasednumber of contacts, the strips may be grouped in diiferent combinations.etc. Additionally the antenna coupling arrangement need not be of thetype shown but may be of the transformer or coupled-coil form in whichthe tuned circuits Ill-A, lli-B, ill-C are associated with magneticallycoupled primary windings for connection directly between antenna andground. If desired autotransformer antenna couplings may also beutilized and have the advantage of a simplicity approaching that of theconstruction illustrated.

The adjusting reactances of the invention that are switched intoresonant circuits for tracking to images may be permanently connected inparallel as an independent adjusting circuit having two terminals. Thistype of adjusting circuit may be used with simpler switching inasmuch asone of its terminals may be kept permanently connected to thecorresponding signal-selecting circuit so that only the other terminalhas to have its connection shifted. The corresponding signal-selectingreactances may also be permanently connected as a parallel circuit withtwo terminals. According to this modification which can be formed fromthe construction of Fig. 1 by merely permanently interconnecting the lowpotential side of the reactive elements of the signal-selecting circuit|4-B for example, the signal-selecting circuit may have its highpotential terminal connected to a switch contact, such as |-3 forexample, and its low potential terminal connected with one terminal of atwoterminal adjusting circuit, to a grounding contact such as l-l2. Thesecond terminal of the adjusting circuit is arranged to be selectablyconnected to the mixer input lead such as the one shown at l-S. Onlythree switch contacts are accordingly needed to shift this type ofcircuit combination instead of the five required in the construction ofFig. 1. The effect of the extra connection or the operation of thecircuit combination is negligible.

As a further modification of the invention. the adjusting reactances mayalso be connected to decrease the resonant circuit inductance and/orincrease the resonant circuit capacitance. Where desired this is readilyaccomplished by connecting the adjusting inductance in parallel with theoriginal inductance, and the adjusting capacitance in parallel with theoriginal capacitance.

The invention is also not confined to thc specic oscillator and/ormixing circuit shown but may he used with any convenient arrangement.

While several exemplifioations of the invention have been indicated anddescribed above, it will be apparent to those skilled in the art thatother modifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:

1. In a multi-band heterodyne signal receiving apparatus, aninterference reducing system including band change switching meanshaving a plurality of contacts, tunable reactance elements for each ofsaid bands connected to said contacts, mechanically coupled tuning meansfor each of said elements for providing unicontrol tuning, heterodynemeans connected to said contacts, a rst circuit including certain ofsaid contacts for selectively connecting said heterodyne means with saidtunable reactance elements of one of said bands, and a second circuitincluding certain others of said contacts for selectively connectinganother of said tunable reactance elements serially in said firstcircuit.

2. In a multi-band heterodyne signal receiving apparatus, aninterference reducing system including band change switching meanshaving a plurality of contacts, radio frequency tuned circuits for eachof said bands connected to said contacts, oscillator tuned circuits foreach of said bands connected to said contacts, a heterodyne meansconnected to said contacts, a first circuit including certain of saidcontacts for selectively connecting said heterodyne means with saidoscillator tuned circuit of one of said bands, a second circuitincluding certain other of said contacts for selectively connecting saidradio frequency tuned circuits of the same band with said heterodynemeans, and a third circuit including certain further of said contactsfor selectively connecting another of said radio frequency tunedcircuits serially in said second circuit.

3. In a multi-band heterodyne signal receiving apparatus, aninterference reducing system including band change switching meanshaving a plurality of contacts, input radio frequency tuned circuits foreach of said bands tunable over a predetermined range and connected tosaid contacts, oscillator tuned circuits for each of said bands tunableat a substantially constant frequency difference above said radiofrequency tuned circuits and connected to said contacts, a heterodynemeans connected to said contacts, a first circuit including certain ofsaid contacts for selectively connecting said heterodyne means with saidoscillator tuned circuit of one of said bands, a second circuitincluding certain other of said contacts for selectively connecting saidinput radio frequency tuned circuit of the same band With saidheterodyne means, and a third circuit including certain further of saidcontacts for selectively connecting another of said oscillator tunedcircuits tunable to a frequency above said radio frequency tuned circuitequal to two times said first named frequency difference serially insaid second circuit.

4. In a multi-band heterodyne signal receiving apparatus, aninterference reducing system including band change switching meanshaving a plurality of contacts, radio frequency tuned circuits for eachof said bands connected to said contacts, oscillator tuned circuits foreach of said bands connected to said contacts, a converter stageconnected to said contacts, a rst circuit including certain of saidcontacts for selectively connecting said converter stage with saidoscillator tuned circuit for one of said bands and tunable to apredetermined frequency range, a second circuit including certain otherof said contacts for connecting said radio frequency tuned circuit ofthe same band and tunable to a frequency range at a substantiallyconstant difference frequency below said predetermined frequency withsaid converter stage, and a third circuit including certain further ofsaid contacts for selectively connecting another of said radio frequencytuned circuits tunable to a frequency range at a substantially constantdifference frequency above said predetermined frequency equal 10 to saidfirst named difference frequency serially in said second circuit therebyproviding an image rejection circuit.

5. Multi-band heterodyne signal receiving apparatus as defined in claim4 wherein said serially connected radio frequency tuned circuitcomprises an inductive branch and a capacitive branch connected inparallel and said third circuit comprises an additional inductanceconnected serially in said inductive branch and an additional capacitorconnected serially in said capacitive branch whereby tracking of saidimage rejection circuit is provided throughout the entire band spectrum.

6. In a multi-band heterodyne signal receiving apparatus, aninterference reducing system including band change switching meanshaving a plurality of contacts, gang-tuned radio frequency circuits foreach of said bands connected to said contacts, oscillator tuned circuitsfor each of said bands connected to said contacts, mechanically coupledtuned means for said oscillator circuit ganged with the tuning means ofsaid radio frequency tuned circuits, a mixer stage connected to saidcontacts, a rst circuit including certain of said contacts connected tosaid mixer stage and said oscillator tuned circuits for connecting saidoscillator tuned circuit of one of said bands tunable to a predeterminedfrequency range to said mixer stage, a second circuit including certainother of said contacts connected to said gang-tuned radio frequencycircuits and said mixer stage for connecting one of said gangtuned radiofrequency tuned circuits of the same band tunable at a substantiallyconstant frequency difference below said predetermined frequency rangeto said mixer stage, and a third circuit including certain further ofsaid contacts and another of said oscillator tuned circuits tunable at asubstantially constant frequency difference above said predeterminedfrequency range equal to said first named frequency difference connectedserially in said second circuit thereby providing an image rejectioncircuit.

7. Multi-band heterodyne signal receiving apparatus as defined in claim6 wherein said serially connected oscillator tuned circuit comprises aninductive branch and a capacitive branch connected in parallel, and saidthird circuit cornprises an additional inductance connected serially insaid inductive branch and an additional capacitor connected serially insaid capacitive branch whereby tracking of the image rejection circuitthroughout the entire band spectrum is maintained.

WEN YUAN PAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name rDate 1,895,809 Macnabb Jan. 31, 19331,896,065 Budenbom Feb. '7, 1933 2,026,075 Wheeler Dec. 31, 19352,137,266 Case Nov. 22, 1938 2,201,938 Albright May 21, 1940 2,281,661Barton May 5, 1942

